Chemistry

posted by .

Why do we have these two equations:

ΔH° = ΣD (bond broken) – ΣD (bond formed)

and

ΔH = Σ ΔHf products - Σ ΔHf reactant

I don't understand when we use which one and why?
Please help. Thanks.

  • Chemistry -

    The second equation gives the delta H for the REACTION as it is written from reactants producing products. The numbers one uses comes from tables which have been prepared from experimentally measured HEATS OF FORMATION (or calculated from different heats of formation using Hess' Law). They are called standard heats of formation and labeled delta Hof and they are at 25 degrees C. They are quite accurate. The values from the first equation CAN be used for the same thing; however, whereas the delta H formation values have a number for each molecule, the bond values for the first equation are averages for C-C bonds, C-H bonds, C-F bonds, etc., from ALL kinds of atoms/molecules. Thus, the first equation gives a quick way of calculating if one doesn't have the formation values handy but they only give approximate values since they are formed from averages.

  • Chemistry -

    That helps. But I still don't understand why we subtract the reactants from the products in one case and when using bonds we subtract the products from the reactants.

  • Chemistry -

    The easiest answer is that "that's the nature of the beast."
    The bottom line is that when reactions take place, we usually go from a higher energy state to a lower energy state. Subtracting as we do gives the correct sign for our "defined" delta H (negative for exothermic reactions and positive for endothermic reactions). We define exothermic and endothermic together with what we define a + delta H and a - delta H to mean.

  • Chemistry -

    Thanks so much!

Respond to this Question

First Name
School Subject
Your Answer

Similar Questions

  1. Chemistry

    Hi! I was studying for an exam (lol) and I came across something I don't quite understand.. could someone help me with Standard Enthalpy of Formation?
  2. Geomtric Series

    I need to find the equations of the following, using Σ (and the n=1 which is at the bottom of the Σ) (and infinity is at the top of the Σ) 1) 5/1 + 8/1 + 1/1 + 14/1 + . . . 2) 6/5 + 9/8 + 14/11 + 21/14 + . . . 3) -6/3 …
  3. chemistry

    # Which of the following is true for the reaction NH3(l) NH3(g) at –33°C and 1 atm pressure?
  4. chemistry

    Suppose the ΔGof, ΔHof, and ΔSo are available and valid at 298 K. Which equation(s) could be used to calculate the change in Gibbs Energy if all product and reactant concentrations (pressures) are 1 M (1 atm) and the …
  5. chemistry

    Nitroglycerin is a powerful explosive, giving four different gases when detonated.2 C3H5(NO3)3 (l) → 3 N2 (g) + 1/2 O2 (g) + 6 CO2 (g) + 5 H2O (g)Given that the enthalpy of formation of nitroglycerin, ΔHf°, is −364 …
  6. Math

    Sorry! I don't get this one either.. limit of (f(x+Δx)-f(x)) / Δ x with Δ x -> 0 f(x)= 1/(x+3) My work so far before I got stuck... 1/((x+Δx)+3) = 1/(x+Δx+3) - 1/x+3 = 1(x+3)/(x+Δx+3)(x+3) - 1(x+Δx+3)/(x+3)(x+Δx+3) …
  7. Math

    The scores of 10 students on their midterm exam (x) and on their final (y) yielded the following data. Σx = 638 Σx2 = 43,572 Σy = 690 Σy2 = 49,014 Σxy = 44,636
  8. Algebra

    I'm trying ti find the correlation coefficient for the following: The scores of 10 students on their midterm exam (x) and on their final (y) yielded the following data. Σx = 638 Σx2 = 43,572 Σy = 690 Σy2 = 49,014 …
  9. statistics

    Scores: 5,2,4,2 ΣX-2 Σ(X-2) Σ(X-2)^2 can you show me a better idea how to solve this thanks
  10. Chemistry

    Calcium chloride, CaCl2, is used in “hot packs”. When the solid is mixed with water, the products are favored. Based on this information, which of the following is true for the values of ΔH and ΔS for the dissolving process?

More Similar Questions